As is known, granules of polymer-based plastic materials that are used for making manufactured products or the like must be subjected to a melting process. Such granules, nevertheless, being hygroscopic, usually contain water molecules, absorbed during previous workings, which when the granules are subjected to melting can enter the molecular chains of the polymers. This leads to the formation of surface defects, blowholes and non-homogeneity both in the manufactured item's structure and color.
Many dehumidification plants were proposed up to now for granular plastic materials. The most used provide for the use of absorbent means or materials, e. g. molecular sieves, which at room temperature have good absorbent properties towards water molecules, while at much higher temperatures demonstrate a substantially opposite behavior, i.e. releasing previously absorbed water molecules.
In such plants, ambient air is made to pass through absorbent means arranged in a container known as “tower” in jargon, in which when the temperature is about 20-25° C. (room temperature), the moisture of the air is absorbed by absorbent means, such as molecular sieves, silica gel, etc. The air thus dehumidified is then suitably heated and made to pass through the granular material to be dehumidified, which gradually loses water molecules contained therein to the hot and dry air hitting it. The duration of the above described process depends on many factors, including: density, granulometry, type of granular material treated and other factors closely connected to the nature of the polymers, as well as to the characteristic features of the employed dehumidification plant.
The absorbent means proposed up to now, nevertheless, have a limited absorption capacity, and if the dehumidification process was achieved by means of a plant comprising a single tower filled with absorbent means, the process could not be continuously carried out, but would be subjected to frequent and prolonged interruptions necessary for desorbing water from the absorbent means.
In order to overcome such drawback, modern dehumidification plants normally use two towers arranged in parallel and are adjusted in a manner such that, alternately, one of the two towers is in regeneration phase, while the other is in process or dehumidification phase of an air current, which, thus dehumidified, will be fed to a hopper containing hygroscopic material. The important feature is that one obtains a continuous production of hot, dry air to be sent to the hopper.
In such plants, when the absorbent materials in the tower in processing phase are about to reach the water molecule saturation state, the operation of the two towers is reversed, by sending hot air into the saturated absorbent means in the tower that was in processing phase, so that the absorbent materials release water molecules absorbed in the preceding phase, while the air to be dehumidified is sent into the tower that was in regeneration phase, whose absorbent material is therefore substantially free of water molecules.
Such dehumidification plants, even if they allow good continuous dehumidification of the granular plastic material to be obtained, require a high amount of energy, particularly required for producing dry air intended to dehumidify the absorbent means.